The present disclosure relates to a mass spectrometer that uses atmospheric pressure plasma as an ion source, and more particularly, to a differential vacuum system used in such a mass spectrometer.
Mass spectrometers that use atmospheric pressure plasma, such as a microwave induced plasma (MIP) or an inductively coupled plasma (ICP), may be used for determining elemental composition of a sample, especially of inorganic materials, such as metallic alloys or geological samples, dissolved in an aqueous solution. In conventional mass spectrometers, the aqueous solution is nebulized into the plasma and resulting elemental ions are extracted into a mass analyzer through an interface, which is a first evacuated region including a sampling cone. A skimmer cone, located behind or downstream of the sampling cone, divides the first evacuated region from a second evacuated region, in which ion optical components, such as extraction electrodes and ion lenses, are situated for maximizing transmission of ions from the plasma into the mass analyzer. The mass analyzer is typically a quadrupole mass analyzer.
The basic principle underlying atmospheric pressure plasma mass spectrometry has not changed, but the technology has evolved over the decades to provide significant improvements, including the provision of tandem (MS/MS) mass spectrometers. An MS/MS mass spectrometer has more than one mass analyzer, typically two quadrupole mass analyzers. The two mass analyzers are separated by a collision/reaction cell into which gas having a relatively small molecular weight, such as hydrogen or helium, is admitted to collide and react with unwanted molecular species which cause spectral interferences. For instance, multi-atomic ions attributable to carrier gas in the sample can be neutralized through the collision/reaction cell, and spectral interference problems otherwise caused by multi-atomic ions in the mass analyzer can be resolved.
An illustrative, experimental implementation of MS/MS mass spectrometry is provided by PCT/GB99/03076 (WO 00/16375), disclosing an inductively coupled plasma mass spectrometer in which a collision cell is employed to selectively remove unwanted artifact ions from an ion beam by causing them to interact with a reagent gas. For purposes of discussion, FIG. 1 herein substantially reproduces FIG. 2 of WO 00/16375. Referring to FIG. 1, a conventional mass spectrometer 110 includes evacuated chambers, containing an ion optical device and a collision cell, divided into first evacuated chamber 106, second evacuated chamber 120 and third evacuated chamber 133. The first evacuated chamber 106 includes an extractor lens 108 and a quadrupole mass filter 117, and is maintained at a high vacuum of about 10−2 mbar to about 10−4 mbar. The second evacuated chamber 120 contains a collision cell 124 pressurized with gas 126, and is maintained at a lower pressure (higher vacuum) than the first evacuated chamber 106, typically about 10−3 mbar to about 10−5 mbar. The third evacuated chamber 133 is arranged after the second evacuated chamber 120, and contains a quadrupole mass filter 137 and a detector 138. The third evacuated chamber is maintained at a lower pressure (higher vacuum) than the second evacuated chamber 120, typically about 10−4 mbar to about 10−6 mbar.
With the provision of the first evacuated chamber 106, the disclosure of WO 00/16375 proposes to reduce the gas load on the collision cell 124 from inductively coupled plasma source 101. To this end, the first evacuated chamber 106 is evacuated by a high-vacuum pump, such as a turbo-molecular pump (not shown), to as low as about 10−4 mbar. This may be the best possible vacuum attainable with the vacuum arrangement disclosed in WO 00/16375, as the first evacuated chamber 106 is separated from the inductively coupled plasma source 101 only by an expansion chamber 103, which is known to be a coarse vacuum stage.
However, the quadrupole mass filter 117 contained in the first evacuated chamber 106 usually requires a relatively high frequency, high voltage AC source of several MHz and several kV, overlapped with DC current on the order of several hundreds of amperes. When the mass spectrometer 110 is operated under the given conditions, discharge may occur at the quadrupole mass filter 117, which could deteriorate or disable the filtering operation and increase background noise. Further, under the aforementioned vacuum conditions, the average free flight distance through the quadrupole mass filter 117 may not be sufficient for some ion species, which could result in insufficient mass selectivity and mass resolution.
Japanese Patent Application No. 2012-001616, entitled “Inductively Coupled Plasma MS/MS Mass Spectrometer” and having a common assignee hereof, attempts to address the above issues. The disclosed Inductively Coupled Plasma MS/MS Mass Spectrometer (ICP-MS/MS) comprises a first vacuum chamber, a second vacuum chamber, a third vacuum chamber, a fourth vacuum chamber, and a fifth vacuum chamber, connected in series. The first vacuum chamber draws sample ions from the atmospheric pressure plasma and passes them to the subsequent stages. The second vacuum chamber includes means for extracting and guiding an ion beam from the ions emanated from the first vacuum chamber. The third vacuum chamber includes a first mass analyzer, such as a first quadrupole mass filter, for selecting and rejecting ions according to a particular mass-to-charge ratio. The fourth vacuum chamber includes a collision/reaction cell into which gas is introduced for removing spectral interferences which would otherwise be caused by multi-atomic ions. The fifth vacuum chamber includes second mass analyzer, such as a second quadrupole mass filter, and a detector, such as an electron multiplier detector. Japanese Patent Application No. 2012-001616 proposes to evacuate the first to fifth vacuum chambers separately and individually. For example, it proposes to evacuate the second and third vacuum chambers separately, so that the third vacuum chamber containing the first quadrupole mass filter can be evacuated to a high vacuum and ions can have a longer average free flight distance.
The vacuum system disclosed in the Japanese Patent Application No. 2012-001616 includes a rotary pump for evacuating the first vacuum chamber, and multiple turbo molecular pumps for evacuating the second through fifth vacuum chambers, which may be backed by the rotary pump. A split-flow turbo molecular pump comprising multiple ports may be used, but in that case the pressure at one port differs from another, and care must be taken such that gas introduced into the collision/reaction cell in the fourth vacuum chamber will not be introduced into the third vacuum chamber due to the pressure difference. When a partial pressure of the gas increases in the third vacuum chamber, there is a risk of discharge or decrease in sensitivity.